The present invention relates to a system and a method for controlling and protecting a foundation brake device of vehicles, particularly trucks, busses and other heavy goods vehicles.
The brake system of a vehicle usually comprises a foundation brake which is a basic drum or disk brake assembly with brake pads fitted to each axle or wheel which produces a braking force necessary to decelerate the vehicle or to bring the vehicle to a stop. If a foundation brake is used excessively, such as if it is applied several times over a prolonged period of time, fatigue or fading, i.e. gradual or sudden loss of braking power, may occur. This is due to the fact that, if brake pads are used excessively, their optimal working temperature is exceeded and their friction coefficient decreases, since certain elements of a brake pad can start to melt and/or vaporise. Furthermore, very frequent brake applications may cause large temperature variations in the brake and hence cause thermal fatigue of the disc.
In order to protect a foundation brake many vehicles are equipped with a supplementary auxiliary brake, e.g. a retarder. A retarder is a device that is permanently fitted to the vehicle's engine or transmission to augment the vehicle's braking capability during prolonged brake application. Such a supplementary auxiliary brake may be used to protect a foundation brake and give the driver greater control and improved brake performance.
Modern vehicles are also often equipped with an adaptive or intelligent cruise control system that can automatically adjust the vehicle's speed to maintain a safe following distance to a vehicle travelling in front or keep the vehicle on a pre-set velocity. An ACC device usually utilises a radar, installed behind the grill of the vehicle, to detect the speed and distance of the vehicle in front. If the vehicle in front slows down or travels slowly or the pre-set velocity is exceeded, the ACC device sends a signal to the engine or braking system of the vehicle to decelerate. Then, when the road is clear, the ACC device re-accelerates the vehicle back to a pre-set speed. Thus, the ACC controls the braking behaviour of the vehicle.
The U.S. Pat. No. 6,044,321 describes such an adaptive cruise control system for a vehicle, wherein applying of a brake is defined by a keeping a defined vehicle-to-vehicle distance. In case the adaptive cruise control decides that a braking operation is necessary for keeping the defined vehicle-to-vehicle distance a retarder is applied and if applying the retarder is not sufficient an automatic brake is continuously operated until the desired vehicle-to-vehicle distance is gained.
Unfortunately, the braking behaviour applied by the adaptive cruise control system according to the state of the art is not applicable to heavy loaded vehicles such as trucks, since the continuous operation of the brakes, particularly of the foundation brakes, easily results in over usage of the vehicles braking system. But also the brakes of all other kind of vehicles show an increased fatigue due to the continuous activation of foundation brakes by an adaptive cruise control system in order to keep the distance to a vehicle in front when driving behind a slow vehicle e.g. in a steep downhill slope.
Therefore, it has been suggested in the patent application WO 2007/078230, to disengage the ACC in case the temperature of the brakes exceeds a pre-set threshold. But, that also means that the driver needs to be informed that the ACC is deactivated and he has to take over control immediately, which reduces the driving comfort and is not desired by the driver.
It is therefore desirable to provide an automated system and method for controlling and protecting a foundation brake in a vehicle which reduce the risk of harmful thermal fatigue problems of foundation brakes.
The invention is based on the general idea that a foundation brake is best protected if frequent or long duration applications of a foundation brake are minimised. For that, the system comprises a control unit which limits the usability of the foundation brake to a predetermined total application-time of the foundation brake within a predetermined time interval.
The total application-time of the foundation brake can be added up from several short application-times of the foundation brake during the predetermined time interval. That means that, for example, if the total application time of the foundation brake is predetermined to 12 s within a time interval of 60 s, the foundation brake can be applied 3 times for 4 s during the time interval of 60 s. This ensures that the foundation brake is not over-used and fatigue and damage due to excessive use of the foundation brake are minimized.
The application of the foundation brakes is preferably initiated if/when the vehicle reaches a minimum distance to a vehicle travelling in front or exceeds a maximum speed. The minimum distance and the maximum speed can advantageously be detected by an adaptive cruise control device, wherein the minimum distance is defined as the distance the vehicle at least needs for not colliding with a vehicle in front, if the vehicle in front suddenly stops or decelerated significantly. The maximum speed is the highest speed the vehicle can travel the road safely. Both minimum distance and maximum speed are dependent on environmental conditions, such as road slope, road conditions, weather conditions, and/or vehicle characteristic conditions, such as vehicle speed, vehicle weight, payload, braking power and/or inter-vehicle conditions, such as inter-vehicle speed.
On the other side, the time between the applications of the foundation brakes is not determined. Thus, the foundation brake can be applied for example for 3 s, released for 15 s, re-applied for 5 s, released for 12 s, re-applied for 2 s, released for 20 s and then reapplied for 2 s, and still easily fulfils the condition of the maximum total application time of 12 s within the predetermined time interval of 60 s.
In a further preferred embodiment, the release-time interval depends on the whether the vehicle reaches the minimum distance to the vehicle in front or exceeds the maximum speed, again. That means, e.g. if the vehicle is running down a slope road, the foundation brakes are applied for some seconds until the distance to a vehicle in front is significantly large and then the foundation brakes are released and the vehicle is allowed to “roll up” to the minimum distance and/or the maximum speed again.
Preferably, during the application of the foundation brakes, the required braking power and thus the application-time and strength of the foundation brake are calculated in order to enable the vehicle to run for a plurality of seconds without re-application of the foundation brakes before the minimum distance and/or a maximum speed is re-reached. The calculation of the required braking power takes also into account environmental conditions, such as road slope, road conditions, weather conditions, and/or vehicle characteristic conditions, such as vehicle speed, vehicle weight, payload, braking power and/or inter-vehicle conditions, such as inter-vehicle speed
The total application-time of the foundation brakes and the predetermined time interval are preferably pre-set by a manufacturer of the vehicle or of the brakes and can be optimised for the vehicle and the vehicle's allowable pay load.
In another preferred embodiment, the maximum total application-time and the time interval are predetermined each time the vehicle starts driving or is powered on. But it is also possible that the driver himself can initiate the determination of total application-time and time interval. This has the advantage that the total application-time can be adapted to the current vehicle condition, e.g. driving with or without pay load, whereby the foundation brakes can be further protected.
In a further preferred embodiment the vehicle further comprises at least one auxiliary brake device which can be applied throughout the deceleration process.
Additionally, the vehicle preferably comprises an adaptive cruise control device having a distance detecting unit for detecting a distance to a vehicle in front and/or a speed limitation unit for limiting the maximum speed of a vehicle. In case a predetermined distance to the vehicle in front is detected or the vehicle exceeds the maximum speed, the auxiliary brakes and/or the foundation brakes are applied in order to decelerate the vehicle.
In case the vehicle in front is very slow or the vehicles travels down a slope road, it is possible that the braking power applied by the auxiliary brakes is not sufficient for the required deceleration. Then, the system can additionally apply the foundation brakes for providing a sufficient braking power.
Further, a cruise control device comprising a preferred embodiment of the inventive system or being adapted to perform an embodiment of the inventive method is preferred.
In the following the present invention will be described in more detail with reference to the accompanying figures. The illustrated embodiments of the figures are exemplary only and are not intended to restrict the scope of the invention thereto. The scope of the invention is defined by the appended claims.